Quantum key distribution involves establishing a key between a sender (“Alice”) and a receiver (“Bob”) by using weak (e.g., 0.1 photon on average) optical signals transmitted over a “quantum channel.” The security of the key distribution is based on the quantum mechanical principle that any measurement of a quantum system in unknown state will modify its state. As a consequence, an eavesdropper (“Eve”) that attempts to intercept or otherwise measure the quantum signal will introduce errors into the transmitted signals, thereby revealing her presence.
The general principles of quantum cryptography were first set forth by Bennett and Brassard in their article “Quantum Cryptography: Public key distribution and coin tossing,” Proceedings of the International Conference on Computers, Systems and Signal Processing, Bangalore, India, 1984, pp. 175-179 (IEEE, New York, 1984). Specific QKD systems are described in U.S. Pat. No. 5,307,410 to Bennett, and in the article by C. H. Bennett entitled “Quantum Cryptography Using Any Two Non-Orthogonal States”, Phys. Rev. Lett. 68 3121 (1992).
The general process for performing QKD is described in the book by Bouwmeester et al., “The Physics of Quantum Information,” Springer-Verlag 2001, in Section 2.3, pages 27-33. During the QKD process, Alice uses a random number generator (RNG) to generate a random bit for the basis (“basis bit”) and a random bit for the key (“key bit”) to create a qubit (e.g., using polarization or phase encoding) and sends this qubit to Bob.
The above mentioned patent and publication by Bennett each describe a so-called “one-way” QKD system wherein Alice randomly encodes the polarization or phase of single photons at one end of the system, and Bob randomly measures the polarization or phase of the photons at the other end of the system. The one-way system described in the Bennett 1992 paper is based on two optical fiber Mach-Zehnder interferometers. Respective parts of the interferometric system are accessible by Alice and Bob so that each can control the phase of the interferometer. The interferometers need to be actively stabilized to within a portion of quantum signal wavelength during transmission to compensate for thermal drifts.
U.S. Pat. No. 6,028,935 (the '935 patent), which patent is incorporated by reference herein, discloses a one-way QKD system that utilizes two unbalanced Mach-Zehnder interferometers located at different ends of the system. There are two main problems with such one-way interferometers used for QKD. One problem involves time variance of the quantum signal polarization. One needs to know the polarization state of the quantum signal precisely as it arrives at Bob's apparatus, otherwise it is very difficult to modulate the signal and keep the interferometer balanced.
The '935 patent tries to address this problem by adding a polarization controller. The polarization controller is needed to ensure a fixed polarization at Bob. However, this adds complexity and expense to the system, as well as unwanted attenuation.
A similar invention to the '935 patent described in Great Britain Patent Application Publication No. GB 2 392063, wherein the QKD system includes a polarization scrambler at Alice to ensure randomness over the Poincare sphere. Note that the scrambling rate needs to be faster than the time rate of change of the PMD of the transmission fiber and faster than the quantum signal detection rate. Also, a polarization scrambler placed at Alice decreases the system's security and adds additional complexity and expense to the system, as well as unwanted attenuation.
Another problem with a one-way QKD system involves temperature drift in the interferometer, which causes a phase shift that destroys the intended interference. In a real-world version of the system of the '935 patent, the sending and receiving stations are separated by a great distance so that they have to be independently thermally controlled, which adds complexity and expense to a commercially viable version of the '935 patent QKD system.
U.S. Pat. No. 6,438,234 to Gisin (the '234 patent), which patent is incorporated herein by reference, discloses a so-called “two-way” QKD system that is autocompensated for polarization and thermal variations. The QKD system of the '234 patent requires that the optical pulses traverse the same optical paths but in different order, which makes the system less susceptible to environmental effects than a one-way system. However, because the photons need to traverse the same path, they must pass twice through Bob's modulator. Thus, as the bit rate goes up, the probability of finding both an outgoing and an incoming pulse passing through the modulator at the same time increases, thereby limiting the system's bit rate.